1. Field of the Invention
The present invention relates to an atomic force microscope.
2. Description of the Related Art
When the pointed tip end of a probe supported on a cantilever is brought very close to the surface of a specimen, a very small attracting or repelling force acts between the atoms of the tip end of the probe and the atoms of the specimen surface. As a result of this interatomic force, the cantilever is curved or displaced, though very slightly.
An atomic force microscope which utilizes the slight displacement of the cantilever is proposed. The atomic force microscope detects the displacement of the cantilever, for the measurement of the atomic force acting between the atoms of the tip end of the probe and the atoms of the specimen surface. The atomic force microscope also allows atom-level observation of the specimen surface by scanning the specimen surface with the probe, with the interatomic force maintained at a constant value. An example of such an atomic force microscope is described in a treatise by G. Binnig, C. F. Quate, Ch. Gerber, et al. (Physical Review Letters Vol. LVI, No. 9 [March 1986], pp 930-933).
Known methods for detecting the displacement of the cantilever includes: a method using a scanning tunnel microscope (STM), an electrostatic capacitance method, a photo detection method, etc.
In the method using a scanning tunnel microscope, a tunnel probe is attached to one side (the obverse side) of a cantilever such that the distance between the tunnel probe and the other side (the reverse side) of the cantilever is short enough to permit a tunnel current to flow therebetween. The displacement of the cantilever is measured on the basis of variations in the tunnel current.
In the electrostatic capacitance method, a plate capacitor is formed such that its one pole plate is constituted by the reverse side of a cantilever (the reverse side being a side opposite to that where a probe is attached). The displacement of the cantilever is measured on the basis of variations in the electrostatic capacitance.
In the photo detection method, the reverse side of a cantilever is formed to have an optically-reflecting face. This optically-reflecting face is irradiated with a laser beam. A variation which the angle of reflection of the laser beam may have in accordance with the displacement of the cantilever and a variation which the interference fringe may have when the laser beam reflected by the optically-reflecting surface is returned to an interferometer, are detected. On the basis of this detection, the displacement of the cantilever is measured.
The present applicants proposed a scanning tunnel microscope combined with an optical microscope. To enable STM measurement within the vision field of the optical microscope, the scanning tunnel microscope comprises a transparent glass plate which is arranged in a plane perpendicular to the observation optical axis of the optical microscope. An STM probe is placed upright on the transparent glass plate in parallel to the observation optical axis, such that the STM probe does not block the vision field of the optical microscope.
The technique of confirming a measurement portion by use of the optical microscope is useful not only to the STM but also to the AFM.
However, if the displacement of the cantilever is detected by providing an STM on the reverse side of the cantilever, the STM largely occupies the vision field of the optical microscope. If the displacement of the cantilever is detected by use of the electrostatic capacitance method, the plate capacitor occupies a certain area of the cantilever, so that the area of the cantilever has to be increased to a certain extent. Accordingly, a large space is required in parallel to the surface of a specimen.
In summary, neither the provision of the STM nor the use of the electrostatic capacitance method is desirable for the detection of the displacement of the cantilever, since the vision field of the optical microscope is largely blocked in either way.